With the recent sophistication in technologies of electronics industries, demands for more miniaturized and integrated devices are increasing. The miniaturization degree in the process technologies have now exceeded the ranges of sub micrometers to reach the order of nanometer(nm). For this reason, devices or components of several tens of nanometers to several nanometers in size are required.
As a microscopic material for producing such components or devices, a carbon nano tube discovered in 1991 is attracting the attention.
Moreover, in recent years, other than the carbon nano tube, ultramicroscopic carbon materials having various shapes such as carbon nano cone, carbon nano wire, carbon nano sheet, carbon nano belt and the like have been proposed or manufactured, and are under research and development for practical application.
As to the carbon material having a graphite structure as described above, there are known a number of synthesis examples for ultramicroscopic structure having a sharp-pointed shape such as carbon nano tube or carbon nano cone having an elongated cone shape.
Generally, as the size of a substance reduces to the order of nanometer, the substance will exhibit a new attribute which is completely different from that when the substance is bulk. For example, a carbon nano tube exhibits an attribute of semiconductor depending on the size or structure, while the bulk graphite is an electric conductor.
When an electric field is applied to a tip end of a nano tube, intense field concentration occurs, and electrons will easily fly into a vacuum from the tip end of the tube due to the tunnel effect.
Because of these properties, the above-mentioned ultramicroscopic carbon materials also attract attention from the view of using as an electronics member.
In production of the above-mentioned ultramicroscopic carbon materials, for example, in order to produce a carbon nano tube, arc discharge from graphite electrode, thermal decomposition of hydrocarbon in gas phase, sublimation of graphite by means of laser and the like approaches have been conventionally used.
In recent years, for example, Japanese Patent Application Laid-Open No. 2000-109308 proposed a method for forming a carbon nano tube film (film formed of a plurality of carbon nano tubes having certain orientation) by applying silicon carbide crystals on a substrate of silicon single crystal by epitaxial growth, followed by etching, heating at high temperatures and the like processes.
Furthermore, Japanese Patent Application Laid-Open No. 2001-48512 discloses a method in which carbon nano tubes are produced directly on a surface of a metal substrate by way of plasma CVD method so as to orient vertically.
Regarding the production or processing method of such ultramicroscopic members or structures, the following two basic concepts are known.
One concept lies in a so-called bottom-up method, wherein using a microscopic substance comprising a molecule, an atom, or a functional group having as same size as molecule or atom, an ion and the like as a material, and the microscopic substance is built up into a desired structure by way of synthesis, modification, transfer, replacement, desorption, migration and the like.
The other concept lies in a so-called top-down method, wherein a macroscopic material (bulk) is processed for size reduction to an ultramicroscopic size region by cutting, milling, decomposing, etching, dissolution and the like.
In the case of carbon nano structures, especially ultramicroscopic structures having a sharp-pointed shape such as carbon nano tube or carbon nano cone as described above, it is practically very difficult to conduct micromachining in the scale of submicrometer or smaller according to the latter method, so that such carbon nano structures are in most cases produced by the bottom-up method.
Besides carbon nano structures, nano cone structures formed of boron nitride (BN), multilayered acicular crystal structures of gallium arsenide (GaAs)/aluminum gallium arsenide (AlGaAs) are also produced in the manner as described above (see Japanese Patent Application Laid-Open No.5-95121).
With regard to silicon (Si), although it is an element belonging to the same group with carbon, synthesis examples for similar nano structures are little known.
Few known cases are at most that a fibrous silicon crystal is incidentally formed by a VLS mechanism (vapor-liquid-solid mechanism) in crystal growth of silicon, and that a fibrous crystal having a stalk-like shape is formed by surface segregation by placing Si microcrystalline particles as seed crystals on a silicon (Si) substrate and heating the surface of the substrate to around the melting point of Si (for example, Japanese Patent Application Laid-Open No. 2002-220300).
On the other hand, regarding the method for sharpening a tip end of a silicon material, a method in which micromachining is conducted by etching is known. However, sharpening the tip end to a nano size by etching is relatively difficult.
One reason for the difficulty in production of a sharp-pointed silicon microscopic structure such as silicon nano tube and silicon nano cone through synthesis is that silicon is easily oxidized in air to change to silicon oxides.
In particular, an ultramicroscopic structure such as nano structure has a very large specific surface area, so that it is susceptible to oxidation and easily oxidized to collide by contact with a trace of oxygen and oxidative substances. A tip end in particular is extremely sensitive to oxidization to readily induce collision of the structure, because the tip end is large in bending torsion compared to the body portion and the like.
Therefore, it was conventionally difficult to produce a nano-sized acicular silicon crystal having a sharp-pointed shape massively and reliably using any of the aforementioned methods.
As a result of diligent studies for solving the above technical problems, inventors of the present invention finally succeeded in forming an ultramicroscopic silicon acicular crystal having a sharp-pointed shape in the course of subjecting a silicon substrate to plasma CVD treatment under a specific condition. Based on this finding, we accomplished the present invention.
It is an object of the present invention to provide an ultramicroscopic acicular silicon crystal having a sharp-pointed shape which is similar to a carbon nano tube or an elongated conical carbon nano cone.
It is also an object of the present invention to provide a method enabling the acicular silicon crystal to be reliably, homogenously and massively formed at a desired location.